There has been a critical temperature indicator of a typical prior art that can be obtained as a commercial name “Monitor Mark” of 3M.
As shown in FIG. 1, the critical temperature indicator 10 includes an indicating material or development material layer 11 to reacted or diffused at or above a predetermined temperature and a wicking member or a development medium layer 12 absorbing a solvent from the development material layer 11. The development material layer 11 is a temperature sensor layer mixed with ink and fatty acid or paraffin, and the development medium layer 12 is made of absorbing papers, non-woven fibers, etc. A barrier or isolation layer 13 is placed between the development material layer 11 and the development medium layer 12. A first major sheet or supporting layer 14 and an opposing second major sheet or both sided-adhesive tape 15 are in turn arranged on the lower portion of the development material layer 11. An indicating layer 16 includes indication windows to be arranged on the development medium layer 12, in which the indication windows includes larger indicating windows 16′ and smaller indication windows 16″ formed in a longitudinal direction thereon. A transparent coating layer 17 is positioned on the right of the indication layer 16.
On the other hand, the development medium layer 12, the indication layer 16 and the transparent layer 17 are provided with cutting portions 18 having the same size, respectively, on the same position of their one side. The temperature indicator 10 further includes a supporting layer 14 attached to the both sided-adhesive tape 15, on which the development material layer 11 and the development medium layer 12 are arranged on the supporting layer 14 with the isolation layer 13 positioned there between. An indication layer 16 is placed on a separation layer 13 with indication windows 16′ and 16″ being positioned on the development medium layer 12. Then, the transparent layer 17 is positioned on the indication layer 16 to finish the assembly of the temperature indicator 10.
Therefore, the temperature indicator 10 is attached to a predetermined position of a refrigeration product through the adhesive portion of the both sided-adhesive tape 15, the cutting portion 18 is removed there from and the isolation layer 13 is drawn out between the development material layer 11 and the development medium layer 12 to be closely contacted with each other. Thereafter, the temperature indicator 10 begins to operate as the development material 11 including a fluid of fat or paraffin with a pigment ink is melted, infiltrated into the development medium layer 12 and immigrated along the lengthwise thereof to indicate the storage state of the products through the indication windows 16′ and 16″.
But the temperature indicator has disadvantages in that the resultant product size is relatively bigger due to the migration of the development material in a longitudinal direction, requires a larger power of attention for removing the cutting layer between the development material layer and the development medium and layer. It is difficult to automate the manufacturing of the product. Since the development material is stored in a state exposed to the air atmosphere without being sealed, it must be refrigerated or cooled for over one to two hours below a melting point thereof before being used. If it is carelessly used and melted at the state exposed to a normal temperature, some fluid of the development material adheres to the isolation layer when the isolation layer is removed. Particularly, the development material is slowly migrated as being gradually far away from the starting point. For it, such like strip type of the temperature indicator has a disadvantage in that an error ranges of the temperature indication time is relatively greater.
Another typical prior art of U.S. Pat. No. 7,232,253 (US Unexamined Publication No. 2004/0240324 published on Dec. 2, 2004) discloses “A time indicator and method of manufacturing same”. The time indicator comprises a first reservoir, a migration medium and activating means for bringing liquid from the first reservoir in contact with the migration medium so that after activation the liquid migrates through the migration medium producing a color change therein. The activating means comprises a second reservoir connected between the first reservoir and the migration medium whereby after activation the liquid travels relatively rapidly from the first reservoir to the second reservoir and then migrates relatively slowly along the length of the migration medium over time.
The patent has some characteristics in that it can make a visible indication of the product life span, but the development material liquefied is migrated to a liquid conduit and then activated in a longitudinal direction with being contacted with the migration medium, so it is resulted in forming the dual paths. It means that it is difficult to measure the exact time elapse. Furthermore, as a weak seal is provided to block the liquid conduit, the activation means of a dish shape portion including a first storing portion must be broken out by a certain force for the operating of a system. However, the weak seal has been often used to be ruptured due to the carelessness of users in operation to draw out the liquid outside therefrom. For it, the time indicator makes the confirmation of the exposed time to a cooling environment become difficult.
Considering these and those points, it is preferable if a critical temperature indicator has an operative mechanism to enable the confirmation of the administration situation from the delivery to the changing of products
Also, it is preferable if it is convenient to use a critical temperature indicator and the critical temperature indicator is easily adapted to a small freeze or refrigerating product and has a simple configuration to enable the mass-production thereof.